vp9: enable multi-thread decoding when refreshctx is equal to 0

vp9: enable multi-thread decoding when refreshctx is equal to 0

In vp9_decode_frame function, ff_thread_finish_setup is not called
when refreshctx is equal to 0, and the next decoding thread can not
start work until the cunrrent frame has been decoded completely. So
ff_thread_finish_setup needs to be called to enable Multi-thread
decoding in this condition.

Signed-off-by: Di Wu <di1028.wu@samsung.com>
Signed-off-by: Reynaldo H. Verdejo Pinochet <reynaldo@osg.samsung.com>